This invention relates to ignition distributors for internal combustion engines having novel rotor electrodes which suppress radio frequency noise generation. More particularly, the invention relates to distributor rotors wherein at least the tip of the rotor electrode comprises a material containing a substantial portion of a dielectric phase interspersed with a conductive metal phase. The dielectric phase protrudes from the metal phase at the tip adjacent to the rotor gap. In the operation of the distributor, electrical phenomena associated with the protruding dielectric phase lead to significant reduction of the voltage required to break down the gap between the rotor electrode and a stationary spark plug lead terminal (hereinafter rotor gap). At such lowered breakdown voltages radio frequency interference is appreciably suppressed.
It is well known that radio frequency interference (RFI) in vehicles powered by spark ignition internal combustion engines is brought about for the most part by high voltage spark discharges across the rotor gap. The source of the RFI signal is the large, fast rise time impulsive current generated at the onset of the rotor gap breakdown. It has been noted that the higher the voltage required to break down the gap, the higher the intensity of the radio frequency interference noise. This is particularly troublesome with modern high energy ignition distributors which at present generally have breakdown voltages at the rotor gap of over 20 kV. We have found that if such breakdown voltage is reduced to less than about 12 kV, the production of radio frequency energy is suppressed and the radio noise substantially reduced.